Fast amplitude and delay measurement for characterization of optical devices

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Abstract

A fast measurement technique based on the modulation phase-shift technique is
developed to measure the wavelength-dependent magnitude and phase responses of
optical devices. The measured phase response is in the form of group delay, which is
used to determine the chromatic dispersion in the device under test by taking the
derivative of the group delay with respect to optical wavelength. The measurement
setup allows both step-tunable and sweeping laser sources. A modulation frequency of
up to 2.7 GHz is accommodated. An alternate method for the phase measurement that
overcomes non-linearities in the measurement setup is also presented. The speed of the
measurement setup is limited by the sweeping speed of the laser source, which for the
Agilent 81682A is 40 nm/sec. The magnitude accuracy is determined by taking a
comparison to the commercially available Micron Finisar measurement system, where
an error of 0.125 dB is noted. The phase accuracy of the measurement setup is tested
by taking the Hilbert transform of the measured magnitude response of an Acetylene
gas cell and comparing it to the integral of the measured group delay. The average
deviation between the two methods is 0.1 radians. An Acetylene gas cell, fiber Bragg
grating, and chirped Bragg grating are tested with the measurement setup and the
Agilent 8168The characterization of the setup leads to the conclusion that the measurement setup
developed in this paper is fast and accurate. The speed of the technique is on the order
of microseconds for a single measurement and excels beyond the speed of the standard
modulation phase-shift technique, which includes measurement times on the order of
minutes. The accuracy of the technique is within 0.125 dB for magnitude
measurements and 0.1 radians for phase measurements when compared to
commercially available measurement systems.2A laser source at 40 nm/sec and the measurement plots are presented.